Self-propelled irrigation apparatus

ABSTRACT

Self-propelled irrigation apparatus for watering non-circular areas includes a main arm assembly pivoted at one end and supported at intervals by self-propelled support towers. An extension arm assembly, mounted on self-propelled support towers having steerable wheels, has one end pivotally connected to the free end of the main arm assembly for irrigating portions of the field outside the circular area traversed by the main arm. Electrical control means are provided to rotate the extension arm assembly relative to the main arm assembly as the latter rotates by turning the steerable wheels of the extension arm. The control means comprises a reversible motor energized by a circuit including banks of switches, the settings of which correspond, respectively, to the angular position of the main arm and the direction in which the steerable wheels are turned.

United States Patent [1 1 Kircher et al.

[ 1 SELF-PROPELLED IRRIGATION APPARATUS [75] Inventors: Robert J.Kircher; Richard D. Setty,

both of Sterling. C010.

[73] Assignee: Sterling Square Sprinkler Company,

Inc., Sterling. C010.

[22] Filed: Oct. 24, 1972 [21] Appl. No.: 300,426

[52] US. Cl. 137/344, 239/212 [51] Int. Cl B051) 9/02, E0111 3/02 [58]Field of Search 137/344; 239/212, 213,

[56] References Cited UNITED STATES PATENTS 2.94l,727 6/1960 Zybach239/212 X 3.352.493 11/1967 Curtis 239/212 X 3.537.470 11/1970Cornelius... 137/344 3,608,826 9/1971 Reinke 137/344 3.623.662 11/1971Reinke 239/213 X 3.703.990 ll/1972 Erickson.... 137/344 X 3.726.4784/1973 McMurray 137/344 X Mar. 19, 1974 3,750,953 8/1973 Reinke 137/344X Primary Examiner--Samuel Scott Attorney, Agent, or FirmDarby 8L Darby5 7 ABSTRACT Self-propelled irrigation apparatus for wateringnoncircular areas includes a main arm assembly pivoted at one end andsupported at intervals by self-propelled support towers. An extensionarm assembly mounted on self-propelled support towers having steerablewheels, has one end pivotally connected to the free end of the main armassembly for irrigating portions of the field outside the circular areatraversed by the main arm. Electrical control means are provided torotate the extension arm assembly relative to the main arm assembly asthe latter rotates by turning the steerable wheels of the extension arm.The control means comprises a reversible motor energized by a circuitincluding banks of switches, the settings of which correspond,respectively, to the angular position of the main arm and the directionin which the steerable wheels are turned.

4 Claims, 10 Drawing Figures 22Y 24Y l POSITION ANGLE ENCODER STEERINGWHEEL1 POSITION I SELF-PROPELILED IRRIGATION APPARATUS This inventionrelates generally to irrigation apparatus, and, more particularly, toself-propelled irrigation apparatus of the center pivot type.

One popular manner of irrigating crops is through sprinkler irrigation.Of the various types of sprinkler apparatus, it has been found that theself-propelled, center pivot irrigation apparatus is the most effectivetype for irrigating large sections of land economically and in a uniformmanner. Self-propelled irrigation apparatus of the center pivot typecomprises an elongated main arm assembly, usually including severalsections connected at their ends, supported at intervals byselfpropelling wheeled support towers. The main arm assembly supports,or may itself constitute, a fluid carrying conduit and includes a largenumber of sprinklers or nozzles spaced along its length. One end of theassembly is pivotally coupled to a base and water supplied to theconduit is discharged from the sprinklers as the assembly rotates aroundthe base, thereby uniformly irrigating a section of land.

One problem inherent in the use of such devices is that the irrigatedsection necessarily takes the form of a circle (with the arm as itsradius). Thus, the use of center pivot type irrigation apparatus withina square section of land will result in a substantial portion of theland (outside that circle) remaining un-irrigated. It has been estimatedthat such systems fail to cover 21.4 percent of the potentiallyagriculturally productive area of a square field.

One method of attacking this problem has been to provide fluid dischargemeans, such as a water gun at the end of the arm assembly facingradially outwardly. Water is discharged through this gun when it facesthe un-irrigated portions of the land. Such systems have not proved tobe entirely adequate, however, since only a relatively narrow additionalarc of land can be irrigated, and, moreover, these water-guns expellarge droplets of water which can cause damage to delicate crops.

US. Patent Application Ser. No. 306545, filed on Nov. 15, I972, in thenames of David Seckler and David Porat, and entitled IrrigatingApparatus, discloses irrigating apparatus capable of irrigating sectionsof a field outside the circular area traversed by the main arm assemblyof a center pivot irrigating apparatus. That invention avoids, orsubstantially minimizes, the drawbacks associated with known water-gunsystems for accomplishing this objective, yet it is simple andrelatively inexpensive. A particular advantage of that invention is thatit may be readily incorporated into existing center pivot irrigationsystems so that replacement of these costly systems is not required.

The present invention is an improvement over the basic invention ofApplication No. 306545, in that it provides a control system for theextension arm assembly which is simpler and more reliable in operationunder the field conditions which the system is likely to encounter.

Briefly, in accordance with the invention, an extension arm is mountedat the free end of a main arm assembly which rotates about a centerpivot. The extension arm carries sprinklers which irrigate areas outsideof the circular area covered by the main arm assembly, and rotates withrespect to the main arm assembly to control the areas which it (theextension arm) covers.

The extension arm is moved by steerable wheels operated by a reversiblemotor under the control of an electro-mechanical switching circuit whichcompares the angle of the main arm with the position of the steerablewheels.

The invention is described in detail below with reference to the annexeddrawings, wherein:

FIG. 1 is a diagrammatic illustration of the basic system as disclosedin Application No. 306545;

FIG. 2 shows diagrammatically the relative positions of the main armassembly and extension arm for a representative scan of one quadrant ofa field;

FIG. 3 is a graph showing steering wheel position as a function of theangle of the main arm assembly;

FIGS. 4A and 4B are front views of the main arm assembly and extensionarm assembly, according to Application No.

FIG. 5 is a plan view of the main and extension arm assemblies ofApplication No. 306545;

FIG. 6 is a sectional view along the line 6-6 of FIG.

FIG. 7 shows one of the encoding wheels used in the embodiment disclosedin Application No. 306545; and

FIGS. 8A and 8B show the preferred embodiment of this invention.

The basic operation of the system is explained with reference to FIG. I.The main arm assembly, shown generally at 12, includes a plurality ofseparate sections 12A,!2B. .N, which are colinearly aligned. Theextension arm is shown at 16 and, for example, may include two sections16X and MY. The extension arm section 16X is pivotally mounted at 17 tothe outer or free extremity of the main arm section l2N.

Each of the main arm sections 12A,B. .N is supported on a respectivesupport tower 18A,B. .N, on which pairs of wheels 20A,B. .N are mounted.The extension arm sections 16X and MY are supported on towers 22X and22Y which contain respective pairs of wheels 24 X and 24Y. Theconstructions of the support towers I8 and 22 are identical except thatthe wheels 24X and 24Y are steerable. In practice, a single steeringmotor shown diagrammatically at 26 may be used to control the wheels 24Xwith the wheels 24X being allowed to follow the wheels 24W eitherpassively or with a power assist.

Water is supplied to the main arm assembly 12 and the extension armassembly 16 both of which include sprinkler devices to distribute thewater over the length of these arms. The main arm assembly may, forexample, be 1,200 feet in length and require anywhere from 24 to 72hours to rotate 360.

Obviously, the main arm assembly 12, which may be considered to be fixedin length, is only capable of covering a circular area of the field. Theextension arm assembly 16, when pivoted about point 17, permitsirrigation of a section of the field outside of this circular area. Bycontrolling the position of the arm 16 with respect to the main arm 12(represented by the angle B), it is possible to irrigate non-circularareas so that in many cases an entire field can be fully irrigated.

Conventionally, irrigating apparatus of the type disclosed isself-propelled in the sense that each pair of wheels 20A, 20B .N, isdriven by a motor (not shown in FIG. I), for example, a constant speedelectric motor. For purposes of explanation, the angular position of themain arm assembly l2 may be represented by the angle a (FIG. I). If itis desired to irrigate a square field, B should vary from 90 (at a toits maximum when a 45 and back to 90 when a 90. This pattern repeats foreach of the successive quadrants in the case of a-square field. Forpurposes of mechanical stability, it is generally desirable that themaximum value of B be less than 180, for example, 140.

As noted previously, the main extension arm consists of a series ofsections 12A,B. .N, which are joined at flexible joints. In prior artconstructions, where there is no extension, the driving motor for theoutermost support tower lSN is driven continuously. A microswitch isplaced at the joint between the outermost section 12N and the nextadjacent section. When the microswitch is actuated because of stress atthe joint caused by movement of the outermost section 12N, the motor forthis next section is actuated.

Similarly, movement of the second section actuates a microswitch at thejunction of the second and third sections to operate the driving motorfor the third section, and so forth for each of the remaining sections.Each of the motors drives its associated support wheels at the same rateof speed and, consequently, since the outermost section llZN travels atthe highest rate of speed, the inner sections would be operatedintermittently with the innermost section operating over the shortestperiods. The system is relatively simple and inexpensive sinceconstantspeed motors, all of which are the same, can be used and specialgear reduction units are not required for the individual sections.

According to the preferred embodiments of the invention, the system iscontrolled by movement of the extension arm rather than the outermostsection of the main arm. The drive motor for tower 18N is notcontinuously operated, but instead, is controlled by a stress switchshown diagrammatically at 29 in FIG. 1.

A position angle encoder 30 is located at the base for the mainextension arm 12. The encoder 30 may be an analog to-digital devicewhich coverts the angle a to a five-bit digital signal. A similarposition angle encoder 31 is physically located at the support tower 22Yfor the steering wheels 24Y, and also produces a five-bit digital signalrepresenting the position angle of the steering wheels of the extensionarm with respect to an arbitrarily selected reference angle (e.g.perpendicular to arm 16).

The two digital signals from the encoders 30 and 31 are coupled to anelectrical comparator 32 which produces an electrical control signalwhen the two digital signals are not equal (or have any otherpreselected relationship). This electrical signal is coupled to thesteering motor 26 which causes the wheels 24Y (and 24X) to turn untilthe encoded digital output representing the steering wheel position isequal (or otherwise corresponds) to the output from the main armposition angle encoder 30. At this position, the steering motor 26 isdeactivated and the extension arm continues to rotate with the steeringwheels in a fixed position.

As the extension arm 16 rotates, a position is reached where the stresson the main extension arm section 12N causes the stress switch 29 to beactuated. When switch 29 is actuated, it energizes the driving motor forthe wheels on the main section ll2N which then causes this section ofthe main extension arm to start moving. In a similar way, as describedin the foregoing, the movement of this outer main section successivelyoperates the drive motors associated with the remaining interiorsections so that the main extension arm sweeps across the field.

In this particular system, it is necessary to determine the direction inwhich the steering wheels must be aligned for each discreet position ofthe main arm assembly. This can be done mathematically if the lengths ofthe respective assemblies and their relative velocities are known. Thedirection of the extension arm steering wheels 24Y for each discreetposition (a) of the main arm is in the same direction as the velocityvector required for the outer point of the extension arm 16 relativeto'the base 10. Hence, this velocity vector is computed for eachdiscreet main arm position and then the encoder wheels (which produce adifferent digital signal for each main arm position) are properlypositioned so as to cause the required control of the steering wheels.By way of example, FIG. 3 shows a workable relationship between theposition angle a of the main arm and the steering wheel position angleqS as indicated in FIG. 2.

It may appear from FIG. 3 that unusually abrupt changes in steeringwheel position are required between 02 40 and a 60 in view of themovement of the main arm assembly; however, the linear velocity of theouter section of the main arm 12 is very low, for example, in the orderof 300 feet per hour. Therefore, for all intents and purposes, theturning of the steering wheels 22Y may be considered to be instantaneousin the sense that the steering wheels are positioned very quickly withrespect to the linear velocity of the assembly.

FIGS. 4A, 4B, 5 and 6, show certain mechanical features of a systemincorporating the basic invention. The parts illustrated in thesefigures have been numbered to correspond with FIG. 1. Inasmuch as theconstruction of the main arm assembly is known, an extended discussionof the physical structure of the system is not included.

Sections 12A,l2B. .N of the main arm assembly are shown as consisting ofhollow conduits through which water is applied to sprinklers 40 spacedalong the individual sections. (Alternatively, a separate conduit forthe water may be supported in conventional fashion onthese sections.)Drive motors 42A, 42B. .N are mounted on respective support towers18A,ll8B. .N to drive the wheels 20A,20B. .N which rotate the main armassembly. As mentioned above, the manner in which these motors arecontrolled is standard, except that operation of motor 42N is determinedby the stress of the section 12N caused by rotation of the extension arm16.

The outermost main arm section 12N may be coupled to the inner extensionarm section MA by a flexible hose 43 which will permit the requiredpivoting of extension arm 16 while supplying water to the extension armsprinklers 44. Support tower 22X for the extension arm section 16)( mayinclude a plate 45 in which a pin 46 extending downwardly from section12N is suitably journaled. Obviously, numerous other satisfactoryarrangements can be used to provide the required pivotable movementbetween these two parts.

The steering arrangement for the steering wheels 24B is shown mostclearly in FIGS. 5 and 6. The steering motor 26 is physically supportedon support tower 22Y above the wheels. Motor 26 is a constant speedelectrical motor which is turned on and off upon receipt of signals fromcomparator 34, as described above with reference to FIG. 1. Thealignment or position of the steering wheels 22Y is controlled by a pin50 connected to vertical connecting rods 52 which rotate axle supports54 on which the wheels are mounted. The physical means for steering thewheels is conventional and, obviously, any suitable arrangement forsteering wheels 22Y may be used.

As the assembly sweeps through any quadrant of the field, as explainedabove, it is necessary that the steering wheels be moved in first onedirection so that the extension arm can start to move outwardly, andthen back in the other direction so that the extension arm can return toits original position. Consequently, in practice, the steering wheelsmust be aligned in the same direction on two separate occasions duringthe sweep of the any given quadrant (i.e. while coming and going). Thiscreates a redundancy or ambiguity which ordinarily would require theelectronic circuits of the comparator to measure the encoded positionangle signals in such a way as to be able to determine the direction inwhich the extension arm is moving relative to the main arm.

According to a further feature of this system, this requirement isavoided by a crank shaft type steering arrangement comprising arotatable disc 56 which rotates in only one direction in response to theactuation of the steering motor 26. The disc 56 is connected to the pins50 by means of a link 58 suitably pinned to the edge of disc 56 (seeFIG. 6) at 59, the other end of link 58 being connected to pins 50 byhorizontal tie rods 60. With this type of arrangement (and properdimensioning and disposition of the steering mechanism), full rotationof disc 56 causes the steering wheel 22Y to rotate 90 in one direction(corresponding to rotation of disc 56 through 180) and then back 90 tothe starting position (as the disc 56 rotates from l80 to 360).Consequently, since there is a discreet position of disc 56 for eachposition of the main arm assembly throughout any given quadrant, thereno longer exists the ambiguity referred to above, although the fullsteering cycle is still available.

A booster pump (not shown) may be associated with the extension arm 16.The booster pump is desirable because the extension arm 16 moves at ahigher velocity relative to ground than the main arm assembly 12. Toassure uniform irrigation, it may be desirable to provide more water tothe sprinklers on the extension arm. The booster pump may be turned offwhen the extension arm is at right angles to the main arm (a 0) and, ifdesired, may be operated to provide a variable water pressure at theextension arm sprinklers, depending upon the position of the extensionarm. This control may be responsive to the encoded position anglesignals in an obvious way. Control of the water pressure at the main armsprinklers and/or sprinkler spacing may be pursuant to conventionalpractice.

FIG. 7 illustrates an encoding wheel which may be used in accordancewith the system as described above. The wheel illustrated in FIG. 7 maybe read by photoelectric means, and suitable devices for this purposeare known. A similar wheel may be used as the steering wheel encoderwith the physical disposition of the respective wheels being properlyadjusted to cause a desired relationahip between the extension arm 16and the main arm 12 for each discreet position of the main arm.

FIGS. 8A and 8B show a schematic diagram for the control circuitry of asecond preferred embodiment of the invention. This embodiment is alsoresponsive to the angle a, but the angle is not measured directly by theencoding means illustrated in FIG. 11. Instead, in the embodiment ofFIGS. 8A and 8B, the angle a is measured by counting the number ofrotations of the wheel 20N of the outermost main arm section 12N. Also,as described below, the steering is controlled by a reversible motor forthe purpose of moving the steering wheels 24 in one direction and thenback in the other. They system of FIGS. 8A and 8B is similar to that ofFIG. 1 except that in place of the encoders and electronic comparatorcircuitry, multi-position rotary switches are used to perform theencoding and comparison functions.

In FIGS. 8A and 8B, the bidirectional motor is shown at M havingterminals M(l) and M(2) which may be selectively grounded. Grounding ofterminal M(l), for example, causes rotation in a clockwise directionwhereas grounding of terminal M(2) will cause rotation in acounterclockwise direction.

In this embodiment, operation is controlled by three banks of rotaryswitches S2, S3 and S4. The bank S2 includes three twenty-four positionrotary switches S2A, 82B and S2C, the armatures of which are controlledby a rotary solenoid Kl. Bank S3 includes four twenty-four positionrotary switches 83A, 83B, S3C and 83D, the armatures of which aremechanically driven in any suitable fashion by the steering mechanismfor the steering wheels 24 of the extension arm assembly.

Bank S4 includes three twelve-position switches 84A, 84B and 84C which,as explained below, function to control the direction of rotation ofmotor M and are stepped by means of rotary solenoid K2.

As shown in FIGS. 8A and 8B, the armatures are shaped differently toaccomplish different switching functions. For example, 538 closes onlyone of its contacts in any position. S3C closes all but one of itscontacts.

In describing the switch banks 81-84, the stationary contacts of eachswitch are identified by the legend corresponding to the particularswitch and a number in parentheses identifying the stationary contact.Thus, S3A(19) is the nineteenth contact of switch SSA.

In the embodiment of FIGS. 8A and 8B, operation is controlled as afunction of the number of rotations of the wheels 20N of the lastsection 12N of the main arm assembly. Thus, as schematically shown inFIG. 8A, wheel 20N may include a cam 60 which operates a cam follower 62to actuate switch S1 once for each revolution of the wheel 2QN. Undernormal conditions, a capacitor C11 is charged by a 28-volt sourcethrough resistor R1. Once during each rotation of wheel 20N, switch S1is actuated by the cam and cam follower 60,62 transferring the charge oncapacitor C1 through diode D1 to rotary solenoid K1. Each time solenoidK1 is energized the switch bank S2 is moved one position in acounterclockwise direction.

The stationary contacts of switch 82B are connected by plugboards ormatrices J5 and J6 to the stationary contacts of switch 53C. Similarly,the stationary contacts of switch 82C are connected by matrices J l andJ2 to the contacts of switch S3D. The individual contacts are connectedby jumpers (not shown in FIG. 8 for purposes of clarity) and, asexplained below, the

manner in which the contacts are connected programs the operation of themotor Ml required to position the steering wheels 24, in a desireddirection for each discrete angular position of the main arm assembly.This programming may, for example, be such as to cause steering wheelposition (4)) to vary with angle a as shown in FIG. 3.

For purposes of example, assume contact S2C(20) is connected to contactS3D(3) by jumpers between matrix J1 and matrix J2. With the circuit inthe illustrated condition, when switch S2C moves to contact S2C(20),terminal M(ll) of motor M is grounded through switches S48, 84C, 53D and82C. Consequently, the motor M is energized, for example, in a clockwisedirection to initiate the rotation of the steering wheels 24. As motor Mrotates, the switch bank S3 is driven and the switches 83A, 83B, 53C and83D are stepped through their successive positions.

When switch S3D reaches contact S3D(3) in the example being considered,the ground connection to terminal M(l) of motor M as traced above isbroken. Consequently, the motor is de-energized and the steering wheels24 come to rest. In this way, the steering wheel position d) asdetermined by the jumper connections between the matrix J1 and matrixJ2.

When wheel 20N completes a second revolution, the system operates in thesame way as described to step 82C to contact S2C(2ll) to again energizemotor M until switch S3D has been stepped to the contact which iselectrically connected to contact S2C(2l) through the matrices Jll,.l2.

When it is necessary to reverse the direction of rotation of the motorto cause steering of the wheels in a counterclockwise direction themotor M1 is placed under the control of the switches 82B and 83C bymeans of the switch bank S4.

Switch S4A has a contact S4A(l) connected to contact 838(25). ContactS4A(2) isconnected to contact S3A(25). The rotary solenoid K2 isconnected to the armature of switch 84A. As with the cases of switches82B, 82C and 83C, 83D, the contacts of switch 83A and 33B are brought tomatrices J4 and J3, respectively, so that any desired one of their 24contacts may be connected to the output terminal 25.

For example, assume that the contact S3B(24) has been connected tocontact S3B(25). Switches SBA and 53B are actuated with switches SSC and53D as motor M rotates as described above. As switch 83B is steppedthrough contacts 838(1) through S3B(23) there is no effect on the systemoperation. However, when contact S3B(24) is closed, solenoid K2 isenergized through switch S4A and switch S3B by the 29-volt d.c. sourcewhich is coupled to the armatures of switches 83A and 838. When switchS5 is in its illustrated position (the function of this switch isdescribed below), solenoid K2 is energized to step the switches S4A, S48and 84C one position, for example, in a counterclockwise direction. Theeffect of this is to connect the terminal M(2) of motor M to contactS4C(2), which in turn is connected to the armature of the switch 53C.Consequently, as wheel N continues to rotate, the motor M11 iscontrolled by the switches SZB and 83C in the same manner as describedabove with respect to switches 82C and 83D except, of course, that motorM now rotates in a clockwise direction. In this situation, when switch83A is rotated so that its armature contacts the contact which isconnected to contact S3A(25) through the matrix J4, the solenoid K2again is energized stepping the switches S4 one position in acounterclockwise direction and returning the circuit to the illustratedcondition.

The switch S5, mentioned briefly above, is under the control of a cam 64which rotates with the rotor of motor M to drive a cam follower 66 whichactuates the switch. A suitable gear train is used to cause a 2440-1stepdown ratio between the cam 64 and the switch bank S3. Consequently,as the armatures of switch bank S3 rotate between any two successivestationary contacts, the cam 64 makes one complete revolution. Duringthis entire revolution of cam 64 (except for the illustrated position),the cam 66 biases the switch S5 so as to ground the armature of switchS48 and switch S4C. This means that the motor M cannot come to restregardless of the positions of switches 83C (or S3D) until the cam 64has rotated to its illustrated position. This position and the operationof the cam and cam follower 64 and 66, respectively, can be adjusted toensure that the switches 83A, B, C and D are precisely centered on eachof their respective stationary contacts. Thus, in a sense, cam 64 andcam follower 66 together with the switch S5 function as a precise switchadjustment to prevent any loss of synchronism.

A switch S6 coupled to a 28-volt d.c. sourcein conjunction with theswitch S2A may be used to manually energize the stepping solenoid Kl sothat the system can be brought to an initial set position.

What is claimed is:

1. Irrigating apparatus for supplying water to a noncircular area,comprising a rotatable main arm assembly for supplying water to acircular portion of said area, an extension arm assembly rotatablymounted at the free end of said main arm assembly for supplying water toportions of said area outside of said circular portion, said main armassembly and said extension arm assembly being supported on towershaving wheels, at least one of the support towers for said extension armassembly having steerable wheels, steering means responsive to theangular position of the main arm assembly relative to said area forchanging the direction of said steerable wheels to thereby rotate saidextension arm assembly relative to said main arm assembly, said steeringmeans comprising a first multiposition switch means responsive to saidangular position of said main arm assembly, a second multipositionswitch means responsive to the position of the steering wheels, and amotor for driving said steerable wheels connected in an energizingcircuit including said first and second multiposition switch means.

2. lrrigating apparatus according to claim 1, wherein said firstmulti-position switch means is actuated by rotation of at least one ofthe wheels of a tower supporting said main arm assembly.

3. Irrigating apparatus according to claim 1, wherein said motor is areversible motor and wherein each of said multiposition switch meanscomprises at least two rotary switches for respectively driving themotor in clockwise and counter-clockwise directions.

4. irrigating apparatus according to claim 3, including two furthermulti-position rotary switches responsive to the position of thesteerable wheels for determining the direction of rotation of saidsteering motor depending upon the position of the steerable wheels.

1. Irrigating apparatus for supplying water to a non-circular area,comprising a rotatable main arm assembly for supplying water to acircular portion of said area, an extension arm assembly rotatablymounted at the free end of said main arm assembly for supplying water toportions of said area outside of said circular portion, said main armassemBly and said extension arm assembly being supported on towershaving wheels, at least one of the support towers for said extension armassembly having steerable wheels, steering means responsive to theangular position of the main arm assembly relative to said area forchanging the direction of said steerable wheels to thereby rotate saidextension arm assembly relative to said main arm assembly, said steeringmeans comprising a first multi-position switch means responsive to saidangular position of said main arm assembly, a second multipositionswitch means responsive to the position of the steering wheels, and amotor for driving said steerable wheels connected in an energizingcircuit including said first and second multiposition switch means. 2.Irrigating apparatus according to claim 1, wherein said firstmulti-position switch means is actuated by rotation of at least one ofthe wheels of a tower supporting said main arm assembly.
 3. Irrigatingapparatus according to claim 1, wherein said motor is a reversible motorand wherein each of said multiposition switch means comprises at leasttwo rotary switches for respectively driving the motor in clockwise andcounter-clockwise directions.
 4. Irrigating apparatus according to claim3, including two further multi-position rotary switches responsive tothe position of the steerable wheels for determining the direction ofrotation of said steering motor depending upon the position of thesteerable wheels.